A clamp is conventionally known, in which a ratchet arm is fixed by shaft to a body having a U-shaped clamp opening having an open upper side, and a conduit is held in the clamp opening by rotating the ratchet arm toward the clamp opening and engaging with teeth provided on the inner perimeter of the clamp opening (see Patent Document 1, page 9, line 6 to page 10, line 4; page 11, line 29 to page 12, line 12; and FIGS. 1 and 7a to 7d).